1. Field of the Invention
Exemplary embodiments of the present invention relate to a data transmission apparatus and method in a wireless communication system; and, more particularly, to a data transmission apparatus and method in a wireless communication system, which uses a carrier sense multiple access with collision avoidance (CSMA/CA) protocol as wireless LAN (WLAN) technology.
2. Description of Related Art
A WLAN standard which has been used the most so far is IEEE 802.11a/g. IEEE 802.11a stations operate in the 5 GHz range, and IEEE 802.11g stations operate in the 2.4 GHz range. The IEEE 802.11a/g WLAN standard may support a maximum data rate of 54 Mbps by modulating a signal through an orthogonal frequency division multiplexing (OFDM) scheme and transmitting the modulated signal at a bandwidth of 20 MHz. Recently, IEEE 802.11n standard has been established to improve throughput. The IEEE 802.11n standard is a high-speed wireless transmission standard which may expand a bandwidth to 40 MHz and may support a maximum data rate of 600 Mbps by using four antennas. The IEEE 802.11n standard should operate in both of the 2.4 GHz range and the 5 GHz range and should maintain backward compatibility with the IEEE 802.11a/g standard.
The WLAN standard uses the CSMA/CA scheme as a media access control (MAC) protocol. In the CSMA/CA scheme, stations connected to one network access point contend with each other to acquire a transmission opportunity for communicating with the access point. Furthermore, the CSMA/CA scheme is a transmission protocol in which a station acquiring the transmission opportunity in such a contention occupies a channel for a predetermined period.
During the period in which the station having acquired the transmission opportunity for accessing the access point occupies the channel, the other stations are switched into a low-power mode or wait in a receive mode until the transmission period of the station having acquired the transmission opportunity is terminated. That is, due to the protocol characteristic of the WLAN standard, only a specific station may communicate with the access point by using a given channel resource during a specific time, based on the competition and avoidance scheme using a back-off algorithm and a carrier sense algorithm. That is, stations which did not occupy the channel in the above-described contention should be in a standby state during a period in which carriers are sensed.
In terms of channel resource use efficiency, the conventional channel utilization method is not effective. The reason will be described as follows.
First, the WLAN standard has 12 channels in the 5 GHz range and seven channels in the 2.4 GHz range, based on a bandwidth of 20 MHz. Furthermore, the 60 GHz range under discussion in the current high-speed WLAN standard is expected to be utilized. It is not common that all the channels are always used for one basic service set (BSS). However, when a specific station occupies a channel, remaining channels are not used.
Second, as the number of stations in a BBS increases, the throughput efficiency of the conventional competition and collision avoidance scheme decreases due to overheads caused by a backoff time, an interframe space (IFS), and a packet header. When a relatively short packet is transmitted during the transmission opportunity period acquired through the contention among a plurality of stations and the channel occupation is terminated, the efficiency is further degraded.
Third, a station acquiring a transmission opportunity is determined on the basis of a simple backoff scheme, without reflecting a link performance with the access point during the transmission opportunity acquisition process. Therefore, when a station a having poor link performance with the access point occupies a channel, it may cause a reduction in the entire performance of the network.